Tarzan and the Ant Men

Tarzan and the Ant Men was published in 1924 and is one of the most unusual books in the entire sequence. After crashing his small airplane in an uncharted forest, Tarzan stumbles into the country of the Alali, a race of giant women who keep their tiny men as slaves and breeding stock. He eventually escapes south into Minuni, a country of two-foot-tall humanoid Minunians who live in walled cities and ride hawks like cavalry, perpetually at war with one another.

The Minunian sections are Burroughs at his most genuinely science-fictional, with worked-out social systems, military tactics scaled for tiny soldiers, and a detailed catastrophic political conflict between the cities of Trohanadalmakus and Veltopismakus. Tarzan is shrunk to Minunian size partway through the novel by a Veltopismakian sorcerer, which lets him participate in the politics firsthand. The mismatch between the ape-man at full strength and the world of tiny soldiers is genuinely funny. A late-period high point of the series.